1. Field of the Invention
The present invention relates to a gearbox comprising at least two parallel shafts carrying gears permanently meshing with one another in pairs, one gear of each pair being connectable to its associated shaft for torque transmission by a friction gear clutch pack which is axially loaded by a ball ramp actuator.
2. Description of the Prior Art
It is known to automate a traditional manual gearbox (transmission) by electronically controlling a plurality of actuators to move the transmission gearshift mechanism in coordination with a fully or partially automated driveline clutch. The actuators move the transmission shifting rails holding the shift forks which in turn control the axial movement of one jaw clutch at the end of each shift fork. The jaw clutch axially slides along a splined transmission mainshaft to engage gearing which nonrotatably couples the input to the output of the transmission. Similar mechanical transmissions are well known in the prior art and may be appreciated by reference to U.S. Pat. Nos. 3,105,395; 3,283,613; 4,754,665, 4,876,924 and 5,053,961, the disclosures of which are hereby incorporated by reference.
It is also known to use ball ramp actuators powered by a separate drive motor for each pair of ball ramp actuators in a transmission to load clutch packs on the mainshaft to frictionally transfer rotary motion from the mainshaft to gears riding on the mainshaft which are in mesh with gears on a parallel countershaft. U.S. Pat. No. 5,078,249, the disclosure of which is hereby incorporated by reference, describes such a gearbox. The ball ramp units consist of two pressure rings and an adjusting ring disposed therebetween. Both the pressure and adjusting rings have variable depth grooves with a rolling member disposed therein for axially expanding and contracting the pressure ring dependent on the direction that the adjusting ring is rotated relative to the pressure rings. The rotary motion of the adjusting ring is supplied by an actuator (motor) that is reacted against the transmission case. To allow the friction clutch pack to rotate with the transmission gear wheel, the pressure ring axially loads the clutch pack through a roller bearing. Thus, the ball ramp assembly does not rotate relative to case ground but only moves through a limited angle as required to cause the rolling member to traverse the variable depth grooves in the pressure ring and the adjusting ring.
The use of an electrical motor type actuator to rotate the adjusting ring relative to the transmission case results in complication and expense associated with the mechanical components. The gearing required to transfer the relatively high speed rotary motion of a motor to the low travel rotary motion of the adjusting gear creates much of this complication. Also, the required roller bearings are expensive.
Another disadvantage with the use of the electric motors to supply the power to the ball ramp actuators is that due to the limited output torque of the motors for a given package size, a gear reduction system must be used to transfer the rotation of the motor to the adjusting ring. This gear reduction slows the speed of response of the ball ramp actuator thereby slowing the speed that a transmission shift can be executed.